CH707189B1 - A magnet assembly comprising a superconducting magnet coil system and a magnetic field shaping apparatus for use in a magnetic resonance spectroscopy apparatus and method of manufacturing the assembly. - Google Patents

Abstract

A magnet assembly for use in a magnetic resonance spectroscopy apparatus having a superconducting magnet coil system (C) for generating a magnetic field in the direction of a z-axis in a working volume (AV) arranged at z = 0, wherein the field of the magnet coil system has at least one inhomogeneous fraction in the working volume A n0 ⋅zn with n ≥ 2, whose contribution to the total field strength on the z-axis varies by z = 0 with the n-th power of z, and wherein a with respect to the z-axis cylindrically symmetric field shaping device (P1) provided of magnetic material is, which at least partially has a radial distance from the z-axis of less than 80 millimeters and compensates at least one of the inhomogeneous field components A n0 ⋅zn of the magnetic coil system to at least 50%, is characterized in that in the field shaping device one or more non-cylindrically symmetric Recesses are provided which are designed so that at least a coefficient A nm or B nm with m ≠ 0 in the magnetic field development of the magnet arrangement according to spherical functions is reduced in terms of amount by at least 50%. Thus, the field homogeneity in the working volume can be increased substantially simply and without increasing the volume of the magnet arrangement, wherein only a few iterations are required to optimize the magnet arrangement.

Description

Description: The invention relates to a magnet arrangement for use in a magnetic resonance spectroscopy apparatus with a superconducting magnet coil system for generating a magnetic field in the direction of a z-axis in a z-axis arranged on the z-axis working volume, wherein the field of the magnetic coil system in the working volume has at least one inhomogeneous portion An0 zn with n> 2, whose contribution to the total field strength on the z-axis by z = 0 with the n-th power of z varies, and wherein a relative to the z-axis cylindrically symmetrical field shaping device of magnetic material is provided, which at least partially has a radial distance from the z-axis of less than 80 millimeters and compensates at least one of the inhomogeneous field components An0 zn of the magnetic coil system to at least fifty percent.

Such an arrangement is known from US 6 617 853 B2.

The field of application of superconducting magnets comprises various fields of application. These include, in particular, spectroscopic magnetic resonance methods. In order to achieve a good spectral resolution in such methods, the magnetic field in the sample volume must have a good homogeneity. With the geometric arrangement of the field-generating magnetic coils, the basic homogeneity of the superconducting magnet can be optimized. Typically, recesses must be provided (so-called Notchstrukturen) in which no wire is wound. Thus, valuable space for magnet windings is lost, which makes the magnet more expensive and increases the stray field.

In an arrangement according to US Pat. No. 6,617,853 B2, a superconducting magnet for high-resolution spectroscopy is made more compact in that one or more magnetic rings are provided, which take over the role of certain notch structures in the magnet coils.

The z-component of the magnetic field of an arrangement according to US 6 617 853 B2 can be developed in the sample volume according to spherical functions:

according to design, the coefficients Anm disappear with m φ 0 and all coefficients Bnm. Due to manufacturing tolerances in the magnet arrangement, the coefficients Anm and Bnm deviate from the calculated value.

Usually Shimspulen are provided to correct these non-vanishing coefficients, which can each be fed with its own power. For large deviations of the coefficients from their nominal value, it may occur that the current required in certain shim coils is too high and the magnetic field of the magnet arrangement can not be corrected as desired. Alternatively, it may happen that the problematic coefficient in the development of the magnetic field due to spherical functions can not be corrected because no shim coil is provided for it. In such a situation, an expensive repair of the magnet system is required in which a part of the magnet assembly must be replaced.

OBJECT OF THE INVENTION The present invention is based on the object of substantially increasing the field homogeneity in the working volume with simple technical measures and without increasing the volume of the magnet arrangement in a magnet arrangement of the type defined at the outset, wherein as few iterations as possible are required to optimize the magnet arrangement ,

BRIEF DESCRIPTION OF THE INVENTION [0008] This object is achieved in a surprisingly simple and effective manner by a magnet arrangement of the type mentioned at the outset, which is characterized in that one or more non-cylindrically symmetrical recesses are provided in the field shaping device, which are designed such that at least one coefficient Anm or Bnm with m φ 0 in the magnetic field development of the magnet arrangement according to spherical functions

is reduced in amount by at least fifty percent.

Due to the geometric arrangement of the recesses can be deliberately certain coefficients Anm or Bnm m φ 0 in the magnetic field development according to spherical functions of the magnet assembly can be changed.

Advantages over the Prior Art: A significant advantage of recesses in a cylindrically symmetric field-shaping device made of magnetic material is the possibility of improving the field homogeneity of the magnet arrangement in the working volume without additional material. In principle, the goal of improved field homogeneity could also be achieved with additional magnetic material which would be glued to the field device, for example. However, this could only be realized who the, if from the outset space would be provided for such field corrections, which would inflate the magnet assembly and make it more expensive.

Preferred embodiments of the invention: [0011] Particularly preferred embodiments of the magnet arrangement according to the invention are characterized in that the field shaping device comprises cooled components, in particular such that they have the temperature of the liquid helium bath which cools the magnet coil system. The advantage of the low temperature is better magnetic properties of the magnetic material, that is, greater magnetization for a given external field. At a stable temperature and fluctuations in the magnetization are suppressed, which ensures a better temporal stability of the Flomogenität the magnet assembly. The homogeneity of the magnet arrangement is also considerably more stable due to the fact that the relative position of the cooled components of the field-shaping device relative to the magnet coil system is not influenced by atmospheric conditions, ie pressure and temperature. Namely, components of the field shaping device that are at room temperature are typically mechanically connected to the magnet coil system over a long distance. This path is deformed by temperature and pressure fluctuations in the laboratory, so that the relative position of these components of the field-shaping device to the magnet coil system is temporally variable. The variable position leads to a temporal dependence of the coefficients of the magnetic field development of the magnet arrangement according to spherical functions.

In a further embodiment, the magnet assembly is characterized in that the field shaping device comprises components which are mounted in a region of the magnet assembly which is at room temperature. These components are easily accessible in the operating state and can be modified without warming up the magnetic coil system.

Particularly advantageous is an embodiment in which the magnetic coil system has an active shield. This active shield reduces the stray field of the magnet assembly, leaving more room in the lab for other applications.

A further preferred embodiment is characterized in that the field-shaping device is arranged at least partially radially within the innermost wire turn of the magnetic coil system. So close to the z-axis, the efficiency of the field shaping device for the compensation of the inhomogeneous field components An0-zn of the magnetic coil system is particularly large.

Also advantageous is an embodiment of the inventive magnet arrangement, in which the field shaping device is magnetically completely saturated and magnetized purely axially (in one direction along the z-axis). In this situation, the calculation of the field produced by the field shaping device is particularly simple and accurate.

In a further advantageous embodiment, the field shaping device comprises components made of soft iron. Soft iron has as advantages a high permeability and a high saturation induction. With these properties, the field shaping device is given a high magnetization, so that a high field efficiency is already achieved with little material.

Also advantageous is an embodiment of the inventive magnet arrangement in which parts of the field shaping device have been subjected to a surface treatment, in particular that they were galvanized. This surface treatment offers optimum protection against corrosion, which is essential in particular for parts made of soft iron.

A particularly preferred embodiment of the inventive magnet arrangement is characterized in that the field shaping device consists of a single element of magnetic material. This is the simplest possible embodiment of the field shaping device in terms of manufacture and assembly.

Also advantageous is an embodiment of the inventive magnet arrangement, wherein the field shaping device comprises a plurality of elements of magnetic material. This offers more degrees of freedom for optimizing the field shaping device.

In a further advantageous embodiment of the inventive magnet arrangement, the field shaping device comprises magnetic films, which are mounted on a carrier device. Particularly close to the z-axis, the efficiency of magnetic material is so great that little material is required to produce the desired field shape. Films therefore offer an ideal solution, not least because they have only slight variations in thickness.

In a further particularly preferred embodiment of the inventive magnet arrangement forms at least a portion of the non-cylindrically symmetric recesses continuous holes through the field shaping device. Such through holes are technically easy to implement, e.g. they can be cut out with laser beams.

Alternative embodiments are characterized in that at least part of the non-cylindrically symmetric recesses does not form continuous holes through the field shaping device. Such non-continuous holes have the advantage of providing more freedom for the design of the field correction. Another advantage results from the fact that the mechanical structure of the field shaping device is weakened less than with through holes, especially when the holes cover a large angular range.

In advantageous developments of these embodiments, it is provided that at least a portion of the non-cylindrically symmetrical recesses is arranged on the inside of the field shaping device. Alternatively or additionally, in other developments at least a part of the non-cylindrically symmetrical recesses may be arranged on the outer side of the field shaping device. Depending on the mechanical production method used, it may be advantageous to remove material either on the inside or on the outside of the field shaping device. With a mandrel for mechanical support on the inside of the field shaping device recesses can be realized with a grinding or milling process on the outside. Spark erosion is easier to achieve on the inside because the electrode is easier to make (mostly in the form of a "cake piece").

In the context of the present invention, a method for producing a magnet assembly of the above-described inventive type, which is characterized by non-cylindrically symmetric recesses (A2, A3, A4) are formed such that at least one coefficient Anm or Bnm with m φ 0 in the magnetic field development of the magnet arrangement according to spherical functions

is reduced in amount by at least fifty percent. Advantageously, at least some of the non-cylindrically symmetric recesses are removed by spark erosion. With spark erosion high mechanical accuracy can be achieved.

Alternatively, it may be provided in another variant of the method that at least a portion of the non-cylindrically symmetrical recesses is removed by a corrosive substance. By suitably covering areas of the field shaping device that do not need to be reworked, material can be easily removed by an etching process in an acid bath. The etching time must be adjusted so that the correct thickness of material is removed.

Another alternative is a variant of the method in which at least a portion of the non-cylindrically symmetric recesses is removed by electrolysis. Instead of an acid bath as in the above process variant, an electrolyte bath is used here.

Finally, at least one part of the non-cylindrically symmetrical recesses can also be removed by means of grinding or milling in a further variant of the method. Grinding and milling are ancient procedures that are mastered by every precision mechanic. There is also no need for any special equipment for carrying out these processes.

In embodiments of the inventive magnet arrangement, which have not cylindrically symmetrical recesses in the form of continuous holes through the field shaping device, the holes can also be cut out with a laser beam. A major advantage of the laser method is the very high mechanical precision, so that even complicated predetermined shapes can be made extremely accurate.

Advantageously, the coefficients to be corrected Anm and Bnm of the magnetic field development are determined by spherical measurements by means of a field measurement in or around the working volume in a magnet assembly with a field shaping device without non-cylindrically symmetric recesses and a suitable geometry of the recesses for correcting these coefficients with a numerical method determined.

Further advantages of the invention will become apparent from the description and the drawings. Likewise, according to the invention, the above-mentioned features and those which are still further developed can each be used individually for themselves or for several in any desired combinations. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWING The invention is illustrated in the drawing and will be explained in more detail by means of exemplary embodiments. It shows:

1 shows a schematic vertical section through a radial half of the inventive magnet arrangement.

FIG. 2 shows a schematic spatial view obliquely from above of an embodiment of the field shaping device according to the invention with not cylindrically symmetrical recesses arranged on the inside and on the outside of the field shaping device; FIG.

Fig. 3 shows an embodiment in which the recesses form continuous holes in the field forming device; Fig. 4 is a schematic development of the field shaping device shown in Fig. 3;

5 shows an embodiment in which the recesses do not form continuous holes but are arranged on the outside of the field shaping device; and

FIG. 6 shows a schematic development of the field shaping device, which is shown in FIG. 5.

1, an embodiment of the inventive magnet arrangement is shown, which comprises a magnetic coil system C and a magnetic field shaping device P1. The field-shaping device P1 is typically at least partially closer to the z-axis than the magnet coil system C. In this case, it consists of 3 rings. On the z-axis, a working volume AV is arranged around z = 0.

FIG. 2 shows a field-shaping device according to the invention, which typically has not cylindrically symmetrical recesses A2 on the inside and on the outside of the field-shaping device. These recesses can in principle take on arbitrarily complicated shapes and depths. In practice, however, simple shapes are preferred because then the influence of the recesses on the field profile of the magnet assembly is easier to calculate.

Fig. 3 shows an inventive field shaping device with continuous recesses for the purpose of changing the B2i coefficient in the magnetic field development of the magnet assembly according to spherical functions. In Fig. 4 is a schematic development of this field-shaping device is shown with Vermassung.

Fig. 5 shows an inventive field shaping device with recesses on the outside of the field shaping device for the targeted change of the B22 coefficient in the magnetic field development of the magnet assembly according to ball functions. In Fig. 8 is a schematic development of this field-shaping device is shown with Vermassung.

To illustrate the invention, two embodiments will now be described in detail. Both examples are based on the same cylindrically symmetric field shaping device, which consists of magnetic steel with a saturation magnetization of 1.71 * 10 A / m. Since the superconducting magnet generates a very high axial field at the location of the field-shaping device, it is assumed that the entire field-shaping device is in magnetic saturation and that the magnetization is purely axial, ie pointing in the z-direction. The field shaping device can be geometrically characterized by its height of 80 mm, wall thickness of 0.5 mm and inner diameter of 70 mm. It is symmetric with respect to the plane z = 0. From this information, the following contributions of the field shaping device to the coefficients of magnetic field development of the magnet arrangement can be calculated by spherical functions: A00 = -100 Gauss A2o = 0.98 Gauss / cm2 A40 = 0.34 Gauss / cm4 All other coefficients are negligible. The contribution to the coefficient A00 is irrelevant, considering that the superconducting magnet generates several tesla. Especially interesting are the positive contributions to the coefficients A20 and A40. Namely, they allow a coil design which produces a negative A20 of -0.98 Gauss / cm2 and a negative A40 of -0.34 Gauss / cm4. This results in compact coil arrays with fewer notch structures than those which would have to produce an A20 of 0 and an A40 of 0. Ideally, even the notch structures can be omitted altogether.

Typically, a field profile is measured in or around the working volume in the test of the magnet assembly with the cylindrically symmetric field shaping device just described. From this, the actual coefficients Anm and Bnm of the magnetic field development of the magnet arrangement according to spherical functions can be determined with a numerical procedure. If any of these coefficients are too large, they can not be zeroed out in the shim procedure, so that a repair of the magnet assembly is required. Using two examples, it should be shown here how an excessively high B21 coefficient or an excessively high B22 coefficient can be corrected by means of recesses in the field shaping device.

The first example is shown in FIG. FIG. 4 shows a schematic development in which φ = 0 ° corresponds to the x-axis. The recesses are in this case through holes through the field shaping device. These holes are symmetrical about the plane z = 0, but offset by 180 °. They all have a width b of 1.85 mm, which corresponds to an opening angle of 3 °. The small hole starts at a z-value Cki of 1.25 mm and has an axial extent hw of 4.5 mm. The large hole starts at a z-value Cgr of 15 mm and has an axial extent hgr of 22 mm. in the positive z-range, the holes are arranged at a mean angle of φ = 90 °, in the negative z-range at a mean angle of φ = -90 °. The contribution of the recesses to the coefficients Anm and Bnm of the magnetic field development of the magnet arrangement according to spherical functions can be determined numerically. The most important non-vanishing coefficients are: B21 = -0.18 Gauss / cm2 A20 = 0.07 Gauss / cm2 A22 = 0.04 Gauss / cm2 [0040] The last two coefficients can be corrected by adjusting the shim currents. The first coefficient is able to compensate for a similarly large contribution from magnet arrangement.

Claims (13)

The second example is shown in FIG. 5. FIG. 6 shows a schematic development in which φ = 0 ° corresponds to the x-axis. The recesses in this case are 0.07 mm deep structures in the field shaping device. Depending on the production process, they can be arranged on the inside or on the outside of the field shaping device. Both recesses are about the plane z = 0symmetrisch and are circumferentially offset by 180 ° to each other. They both have an opening angle of 90 ° and an axial height of h = 56 mm. The contribution of the recesses to the coefficients Anm and Bnm of the magnetic field development of the magnet arrangement according to spherical functions can be determined numerically. The most important non-vanishing coefficients are: A22 = -0.58 Gauss / cm2 A20 = -0.64 Gauss / cm2 The first coefficient is able to compensate for a similarly large contribution from magnet arrangement. The second coefficient can be corrected by adjusting the corresponding shim current. If the shim is too weak for this, additional pits may be provided in the field shaping device which correct this coefficient. However, these depressions are cylindrically symmetrical in contrast to the recesses. claims A magnet assembly for use in a magnetic resonance spectroscopy apparatus having a superconducting magnet coil system (C) for generating a magnetic field in the direction of a z-axis in a z-axis arranged on the z-axis working volume (AV), the field of the magnet coil system (C) in the working volume (AV) has at least one inhomogeneous portion An0 zn with n> 2, whose contribution to the total field strength on the z-axis varies by z = 0 with the nth power of z, and wherein one with respect to z Axis cylindrically symmetrical field forming device (P1; P2; P3; P4) of magnetic material is provided, which at least partially has a radial distance from the z-axis of less than 80 millimeters and at least one of the inhomogeneous field components An0 zn of the magnetic coil system (C) to at least fifty percent compensated, characterized in that in the field shaping device (P1; P2; P3; P4) one or more non-cylindrically symmetrical recesses (A2; A3; A4) are provided, which are formed so that at least one coefficient Anm or Bnm with m φ 0 in the magnetic field development of the magnet assembly according to ball functions is reduced in amount by at least fifty percent. 2. Magnet arrangement according to claim 1, characterized in that the field shaping device (P1; P2; P3; P4) comprises cooled components, which in operation in particular have the temperature of a liquid helium bath which cools the magnetic coil system (C). 3. Magnet arrangement according to claim 1 or 2, characterized in that at least part of the non-cylindrically symmetrical recesses (A2; A3; A4) forms continuous holes through the field shaping device (P1; P2; P3; P4). 4. Magnet arrangement according to claim 1 or 2, characterized in that at least part of the non-cylindrically symmetrical recesses (A2; A3; A4) does not form continuous holes through the field shaping device (P1; P2; P3; P4). 5. Magnet arrangement according to claim 4, characterized in that at least a part of the non-cylindrically symmetrical recesses (A2; A3; A4) is arranged on the inside of the field forming device (P1; P2; P3; P4). 6. Magnet arrangement according to claim 4, characterized in that at least part of the non-cylindrically symmetrical recesses (A2; A3; A4) is arranged on the outside of the field shaping device (P1; P2; P3; P4). 7. A method for producing a magnet assembly for use in a magnetic resonance spectrometry apparatus with a superconducting magnet coil system (C) for generating a magnetic field in the direction of a z-axis in a on the z-axis z = 0 arranged working volume (AV), wherein the field of the magnetic coil system (C) in the working volume (AV) has at least one inhomogeneous component An0 zn with n> 2, whose contribution to the total field strength on the z axis varies by z = 0 with the nth power of z, and where cylindrically symmetrical field-shaping device (P1; P2; P3; P4) of magnetic material is provided which has at least partially a radial distance from the z-axis of less than 80 millimeters and at least one of the inhomogeneous field portions An0zn of the magnet coil system (FIG. C) compensated for at least fifty percent, characterized in that in the magnetic field shaping device not cylindrically symmetrical Ausspa ments (A2; A3; A4) are formed such that at least one coefficient Anm or Bnm with m φ 0 in the magnetic field development of the magnet assembly according to spherical functions is reduced in amount by at least fifty percent. 8. The method according to claim 7, characterized in that at least a part of the non-cylindrically symmetrical recesses (A2, A3, A4) is removed by spark erosion. 9. The method according to claim 7, characterized in that at least a portion of the non-cylindrically symmetrical recesses (A2, A3, A4) is removed by a corrosive substance. 10. The method according to claim 7, characterized in that at least a portion of the non-cylindrically symmetrical recesses (A2, A3, A4) is removed by electrolysis. 11. The method according to claim 7, characterized in that at least a portion of the non-cylindrically symmetrical recesses (A2, A3, A4) is removed by means of grinding or milling. 12. Method according to claim 7, characterized in that the continuous holes in the field shaping device (P1; P2; P3; P4) are cut out with a laser beam. 13. Method according to one of claims 7 to 12, characterized in that the coefficients Anm and Bnm of the magnetic field development for spherical functions to be corrected are determined by means of a field measurement in or around the working volume (AV) in a magnet arrangement with a field shaping device (P1; P2; P3; P4) before forming the non-cylindrically symmetrical recesses (A2; A3; A4) and determining the suitable geometry of the recesses for correcting these coefficients with a numerical method.